The third-order optical nonlinearities of methyl green in aqueous solution are studied by the Z-scan technique using a femtosecond laser at 800 nm. The valley shape in the open aperture measurements reveals the presence of reverse saturable absorption effect, which is ascribed to the three-photon absorption process. The closed aperture Z-scan traces demonstrate a negative sign for the nonlinear refractive coefficient, indicating the self-defocusing optical nonlinearity. Furthermore, the magnitudes of the nonlinear refractive index n2, the three-photon absorption coefficient α3, and the real part of the third-order nonlinear optical susceptibility Re χ(3) are determined and found to be linearly proportional to the dye concentration within the investigated range. Our results suggest that methyl green may be a hopeful candidate for applications in nonlinear optics.